Until now the materials used in these structures are wood, metals, reinforced concrete, and fiberglas reinforced polyester.
When wood is used, one starts with conifers of a minimum of fifteen years in order to obtain a height of nine meters under favorable conditions. After felling, removing the branches and letting it dry, it must be debarked and cleaned, later placing it in an autoclave which at 120.degree. will produce its sterilization and creosoting.
The structures thus obtained burn very easily, are not homogeneous either in their form or appearance, from 25.degree. on they spot, produce sparks when being climbed, if the phreatic levels are high the moisture of the ground decomposes them at their embedded section, the termites and the carpenter birds easily destroy them, when it rains they soak up water and lose their insulating quality, and lastly in warm climates they open up and splinter.
Wooden posts or structures have for many centuries been an easy solution, not requiring any technological effort other than to check their natural tendency to self-destruction.
On this material there is no technical system for calculating its strength or its bending moments, only empirical tables.
If the use of metals is taken into consideration (iron, aluminum, stainless steel), it is desirable to narrow the selection down to iron, as others are too costly for installations of the kind referred to.
Iron structures are suitable for heights greater than sixteen meters, especially when they are of latticework. They must be hot-galvanized, including the screws and bolts.
For smaller structures (as used in urban centers), their conductivity presents a problem, so that for safe electric wiring functional grounding must be provided.
Besides galvanizing, the application of coats of paint over the zinc, to be renewed periodically, is desirable in many cases.
If instead of using the latticework structure, they are constructed from bent sheet iron or tubular iron, it becomes impossible to follow exactly the resisting moment at each height, so that their deformation under stress is irregular, and in the former case one depends on the quality of the welds that have been reduced and filed down to obtain an acceptable appearance.
On the other hand, the post acts as a chimney and produces condensation in its interior, which is inaccessible, this being aggravated in saline environments with the corrosion that goes from the interior to the exterior without any possibility of stopping it by subsequent treatments.
Lastly, one of the greatest drawbacks in the use of iron in urban centers is the destruction that occurs where the post and the ground come together, where moisture always accumulates, reinforced by the contributions of animal urine, which inevitably cause the corrosion of the galvanized iron.
In the use of reinforced concrete (not prestressed and vibrated), a series of metallic reinforcements are needed, formed by thick rods of steel, joined by stirrups which ensure their position, which are embedded by a mortar of cement and aggregates, being vibrated before setting to expel the pockets of occluded air, many hours are needed for its presetting, at which time the form necessary for each unit can be extracted, its nominal strength being reached after several days.
The rods must be covered with at least 2.5 cm of the concrete itself, this part remaining in the zone of the tensile stresses, and this produces cracks which reach the metallic reinforcement and cause its oxidation, thereby increasing its volume to the point of bringing about its self-destruction. Besides, the use of reinforced concrete is not desirable in the production of structures subjected to bending: because of its excessive weight, because of the risk of violent rupture, because of the lack of homogeneity in the composition of the concrete when adding to it aggregates difficult to control, and because of the difficulties which its handling presents.
In the use of centrifuged and prestressed reinforced concrete techniques are employed which require large installations, as well as machinery with a power in the order of some 100 HP for each form.
The product obtained, although of better quality than the above (not prestressed and vibrated), has the great disadvantage that by the centrifugation a classification of the aggregates according to their density takes place. Other disadvantages are the manual preparation of the reinforcements, and that the presetting time must be shortened as much as possible to be able to prestress and strip, so that it is necessary to heat the entire assembly and to add chemical accelerators for the setting of the cement, which diminishes its quality.
When the structure obtained is prestressed, the cracks produced by its bending in the bottom part are less as the deflection at its upper end decreases, but they still exist, so that it is normally necessary also to paint them. The cost is high, the production troublesome, the wait to complete setting long, and its transportation painstaking and cumbersome.
The use of polyester and fiberglass in the manufacture of these structures has been the subject of much attention in recent years, in industrialized countries, in search of an insulating product resistant to atmospheric agents and which does not require preservation.
This material offers, in the case considered, very notable disadvantages, such as: the lack of rigidity; the flammability of the product, which burns and propagates fire; the low impact resistance, the shock of a vehicle can break a post; the very high cost; the difficult stabilization to sunlight; the very difficult adjustment of its resistant cross section in accordance with each bending moment, as one obtains by centrifugation that it cuts very easily; the feeling of provisionality for lack of mass and solidity.
From all this it appears that in this sector neither the material nor the structure suitable for finding a solution to the technical exigencies of service and safety have been found; actually nothing more has been done than to make use of the various products on the market, intended for other applications.